Pterotrachea hippocampusPhillipi 1836

Introduction

The body morphology of Pterotrachea hippocampus is elongate; similar to that in P. coronata but not as streamlined (see the P. coronata page). Instead, the proboscis is shorter and thicker, the visceral nucleus is shorter and pyriform (teardrop) shaped, and the tail is shorter and less well developed. The eye shape is unique in the genus, with a narrowly- to broadly-triangular shape. The trunk cutis anterior to the swimming fin is thickened ventrolaterally, forming a bib, and the tail terminates in two small leaf-like lobes, as in P. coronata. Like the other species of Pterotrachea, a swimming fin sucker is present only in males. The geographical distribution is cosmopolitan in tropical to subtropical waters.

Brief Diagnosis

A species in the genus Pterotrachea with the following characteristics:

Viewed dorsally, eyes narrowly (juveniles) to broadly (adults) triangular in shape. Change in shape due to lengthening of the retinal base (compare the three photographs below), which can be quantified by the ratio of the eye length to retinal base width. In Hawaiian P. hippocampus this ratio decreases from 1.6 in juveniles to 1.0 in adults (Seapy, 1985)

Visceral nucleus pyriform, or teardrop, shaped. In Hawaiian specimens (Seapy, 1985) the length to width ratio averages 2.9 and decreases gradually with age (Seapy, 1985). This average value is nearly identical with that for P. scutata (3.0) but differs dramatically with that for P. coronata (4.8)

Comments

Taxonomy: The validity of Pterotrachea minuta Bonnevie 1920 was not questioned until a morphological analysis of the species in the genus Pterotrachea from Hawaiian waters was undertaken by Seapy (1985). Specimens tentatively identified either as P. hippocampus or P. minuta over a broad size range revealed a continuum of change in the shape of the eyes and the visceral nucleus (the two primary criteria used historically to distinguish the two species). Regardless of the initial tentative species identification, young individuals possessed narrowly triangular eyes and longer visceral nuclei which, with growth, transitioned to broadly triangular eyes and shorter visceral nuclei in older adults. The conclusion of the study was that there was no justification for recognizing P. minuta as a valid species; at least in the North Pacific Ocean. A subsequent morphological comparison of specimens collected from the North Atlantic Ocean that had been previously assigned to P. minuta and P. hippocampus was undertaken by Seapy (2000). In addition to eye and visceral nuleus shape, all of the other characters cited by Bonnevie (1920) to distinguish the two species were examined. The results confirmed and amplified upon those of the earlier Hawaiian study.

Ecology: Among the heteropods collected during the Meteor Expedition in the northwestern Indian Ocean (Richter, 1974), Pterotrachea hippocampus was rare; only four individuals were recorded. From the Amsterdam Mid North Atlantic Plankton Expedition (Pafort-van Iersel, 1983), a total of 72 P. hippocampus were captured, which was comparable with the total for P. scutata (74) but only one-half the number of P. coronata (146). Opening-closing midwater trawl tows were made along a north-south transect (approximately at 30°W longitude) between 25°N and 49°N. Nine individuals were collected between 36°N and 49°N, while the rest were from 25°N to 35°N. Although extending somewhat into subtemperate waters, these records support the general observation that the P. hippocampus is a tropical to subtropical species. In a diel vertical distribution study of heteropods from Hawaiian waters of the North Pacific (Seapy, 1990), P. hippocampus was the second most abundant heteropod species (20% of the total), behind Atlanta lesueurii (29%). However, in a subsequent study from Hawaiian waters (Seapy, 2008), P. hippocampus was recorded in very low numbers; ranking eighth of twelve species (mean nighttime density = 1.2% of the total) in a fall sampling period and eleventh of seventeen species (mean nighttime density = 0.2%) in a spring period. The striking difference between the abundances in the two studies suggests that the species' abundance in Hawaiian waters is highly variable.

Vertical distributions of carinariids and pterotracheids in the central North Atantic were reported by Pafort-van Iersel (1983) based on samples collected with paired 1-m2 and 8-m2 opening-closing midwater trawls. Pterotrachea hippocampus (data for P. hippocampus and P. minuta combined) was mainly recorded from the upper 200 m (eleven tows and 50 specimens); below 200 m only ten specimens were captured by six tows. These results suggest that the species is primarily epipelagic (upper 200 m), although its range extends into the mesopelagic zone in low numbers. Also, there was no evidence for nocturnal vertical migration by individuals below 200 m since equal numbers were recorded from the day and night tows. In Hawaiian waters the large numbers of P. hippocampus collected in the study by Seapy (1990) provided a large data set with which to investigate diel vertical distribution patterns in the epipelagic zone (0-400 m). The species ranged downward to 300 m during both day and night periods, although the abundance below 200 m was low. During the day the highest mean densities were at the 90-140 m and 140-200 m depth intervals, while at night the highest densities were in the 0-45 and 45-90 m intervals. These data support an hypothesis of nocturnal vertical migration into the upper 90 m by a large portion of the population living between 90 and 200 m during the day.

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Pterotrachea hippocampusPhillipi 1836.
Authored by
Roger R. Seapy.
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